The billboard, located off the I-93 expressway that crosses the city, belongs to the local chapter of the International Brotherhood of Electrical Workers (IBEW) union. The chapter, Local 103, usually uses the billboard for union-related and community messages but that changed Tuesday morning, less than 24 hours after the two bombs exploded.

None of the union's 7,500 members were directly hurt in the explosions, according to a representative, but they have another connection.

The town where the chapter's headquarters is located, Dorchester, Mass., is the same town where the blasts' youngest victim, Martin Richard, and his family lived. Richard, 8, died while attending the marathon festivities with his mother, father and two siblings. Both his mother and younger sister sustained "serious" injuries in the attacks.